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Nowhere to hide – invisibility cloaks fail near light speed

Now you see it… now you don’t

Colin Hawkins/Getty

By Jacob Aron

Engage the cloaking device! Fire up the warp drive! Wait, how come the enemy ship can see us? It turns out that invisibility cloaks stop working completely when travelling near light speed, and even smaller velocities can give you away.

That reads a bit like a line from a Harry Potter/Star Trek mash-up, but researchers have been working on real invisibility cloaks for a while now. The devices draw on a theory called transformation optics, which allows for the design of materials that can bend the path of light around them, hiding anything inside from view. While full-blown cloaks don’t exist yet, there has been some progress in hiding small objects at certain frequencies.

“If you have an ideal cloak in front of you in a room, it’s hard to tell if there is an object with a cloak around it, or if there is nothing,” says Jad Halimeh at the Ludwig Maximilian University of Munich in Germany.

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Although the cloak looks like a patch of empty space, Halimeh and his colleagues wondered if there was a way to distinguish between the cloak and nothingness.

It seems that the answer is yes: Albert Einstein’s theory of relativity spoils the fun. That’s because relativistic effects kick in as you approach the speed of light, preventing the cloak from working. “If I’m moving with respect to you, what I see as space and time is different from what you see,” he says.

Speed of light

The team studied a simulated cloak capable of hiding a single frequency of green light, and modelled it moving at various speeds in a laboratory illuminated by that frequency. They found that just 1 per cent of the speed of light (around 10 million kilometres per hour) was enough to start ruining the cloaking effect. The light’s frequency decreases if the cloak is moving away from you or increases if it’s moving towards you, similar to the way an ambulance siren’s pitch changes as it drives past you. That means the light is no longer at the cloak’s operational frequency, revealing its presence. The effect is even more extreme at higher speeds.

“You start seeing something fishy,” says Halimeh. “It’s as if you see some ghost emerging in front of you, popping out of nowhere into existence.” In a follow-up paper, Halimeh studied cloaks that operate at multiple frequencies, and found that a slightly different effect also prevents them from working at relativistic speeds.

Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, says he always expected invisibility cloaks would struggle to keep up at high speeds. “What I found surprising was that one needs rather larger velocities, a per cent of the speed of light in a vacuum, to see significant adverse effects,” he says. “It reminds me of the driver who claimed to have seen the red traffic light shifted to green, and was fined for relativistic speeding.”